Decimal -> Simplified Fraction in Javascript

fracs.js

// This was made to convert shutter speeds from node-exif from decimals of a second to fractions of a second.
// Example: 0.00125 converts to 1/800


var input = "0.00125";

function doMath(result, string) {
    console.log(`${string} is ${result[0]}/${result[1]}th of a second`);
    return result;
}

function convertShutter(string, callback) {
    var input = string;
    var length = string.length;
    var index = [];

    for (i = 0; i < length; i++) {
        var theIndex = input.substr(i, 1)
        index[i] = theIndex;
    }
    var arrLength = index.length;
    var partOne = []; // Zeros
    var partTwo = []; // Numbers larger than 0
    for (i = 0; i < arrLength; i++) {
        var value = index[i];
        if (value <= 0) {
            partOne += value
        } else if (value === '.') {
            partOne += value
        } else if (value > 0 && value != null) {
            partTwo += value
        } else {
            console.log(`${value} at ${i} did not fall in a category!`)
        }
    }

    var arr1Len = partOne.length - 2;
    var arr2Len = partTwo.length;

    var power = Math.pow(10, arr1Len + arr2Len);

    // Factoring Numerator, the first index of the second array (125 in this case)
    var stepOneNumerator = partTwo;

    // Take the power (100,000) and divide by the numerator (125) to get 800 (125 goes into 100,000 800 times)
    var factoring = power / stepOneNumerator

    // result[0] = Numerator, result[0] = denominator (result = regular shutter speed)
    var result = [1, factoring];

    callback(result, input);
}

convertShutter(input, doMath);
// OUTPUT: 0.00125 is 1/800th of a second